DC-DC converters are power electronic converters which convert a fixed DC voltage to a variable DC voltage.
Buck-Boost converter is a DC-DC converter which produces a DC output voltage greater than or less than the input DC voltage.
Read about buck converter/stepdown converter
Read about boost converter/step down converter
Buck-Boost converter produces a output voltage which is opposite in polarity to the input voltage. Hence buck-boost converter is called as inverted output converter.
Operation of buck-boost converter
The buck-boost converter consists of a controlled switch, a diode, an inductor and a capacitor.
Ton – time for which switch is ON
Toff – time for which switch is OFF
Total time T= Ton + Toff
Duty cycle D= Ton/ T
The value of duty cycle D ranges between 0 and 1.
When the switch is ON, the inductor stores the energy. The input source current is equal to the inductor current. As the input is isolated from the output side, the capacitor supplies the load current.
Find the inductor voltage and capacitor current equation
When the switch is OFF, the energy stored in the inductor is released to load.
The inductor voltage over a time period is zero under steady state conditions.
where D is called the duty cycle D.
The negative sign is used to show that the output voltage polarity is opposite to the input voltage polarity.
When D< 0.5, buck converter
When D> 0.5, boost converter
In steady state, the average capacitor current over a time period T is always zero.
The inductor current ripple can be obtained from either ON or OFF condition of the switch. Here ON condition of the switch is considered to find the current ripple. It can be observed that current ripple is inversely proportional to the inductance and frequency. As frequency increases, filter size decreases.
the inductor current ripple is same as the boost converter.
There are two modes of operation in dc-dc converters. Continuous conduction mode and discontinuous conduction mode. In continuous conduction mode, the inductor current remains positive while it becomes zero for a short period in discontinuous conduction mode. Critical inductance- minimum inductance required to maintain continuous conduction mode of buck-boost converter. If the chosen inductance is less than the critical inductance, the converter may go to discontinuous mode otherwise it may remain in continuous conduction mode.
The output voltage ripple is mainly reduced by the capacitor. Normally, for converter design, the percentage of ripple allowed will be given. The charge stored on the capacitor can be used to calculate the output voltage ripple. The charge on the capacitor during the ON period of the switch is a rectangle. The area of the rectangle ( b*h) gives the stored charge.
the output voltage ripple is same as the boost converter
Buck-Boost converter- step up/down converter
Output voltage = Vs D/ (1-D)
Inverted output converter- Vo polarity is opposite to Vs
The input and output are not connected at any time.
Voltage ripple “ΔVo “ and current ripple “ΔIL” are same as boost converter.
Critical inductance = D* Lc of boost converter
Power Electronics. Converters, Applications and Design (Ned Mohan, Tore M. Undeland, William P. Robbins), John Wiley and Sons, Inc, 2003.
Fundamentals of power electronics,(Robert Warren Erickson, Dragan Maksimović), Springer, 2001.
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